Optical device for pilot&#39;s visor comprising a tubular mirror

ABSTRACT

An optical device for a system presenting collimated images using a spherical mirror. The optical device makes it possible to present the user with an image corrected of off-centring distortion of a second kind due to an off-axis spherical mirror. In order to do this the optical device includes a tubular mirror whose optical characteristics ensure a high-quality image and correction of the off-centering distortion. The tubular mirror has a surface generated by the translation of a plane curve (a circular arc) along another plane curve (also a circular arc). The optical device is especially applicable to helmet sights for an aircraft pilot.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical device for correctingaberrations affecting an image. In particular, a device according to theinvention enables the distortion due to a spherical concave mirrorinclined with respect to the direction from which this mirror isobserved to be corrected.

The invention is applicable especially, but not exclusively, to a helmetsight for the pilot of combat aircraft or helicopters or for theoperator of a training simulator.

2. Discussion of the Background

A helmet sight is an image presentation device integrated into a helmet.The sight enables the person wearing the helmet, for example the pilotof an aircraft in flight, to observe visual information at the same timeas viewing the scenery, or the cockpit, which he perceives most oftenthrough a protective visor.

The presentation of appropriate information, for example in the form ofsymbols, provides assistance to piloting and navigation. So, for armedvehicles, the presentation of a reticule provides assistance when aiminga weapon.

The information may also consist of an image of the scenery acquired bysensors other than the eyes of the helmet wearer, such as infrared imagesensors or light intensifiers to complement or replace direct vision.

Inside the helmet, an image generator comprises an imager whose screen,for example a cathode-ray tube screen or a liquid-crystal screen,enables an image to be displayed.

The image is most often conveyed using an optical relay system up to acombiner which ensures that the conveyed image is presented superimposedon the view of the scenery.

To enable the pilot to simultaneously observe the scenery vieweddirectly at infinity and the image from the imager, the image is alsobeing focused at infinity by an optical collimation system.

When the combiner is formed from a simple semi-reflective flat plate,the image may be collimated by an optical system placed between theimager and the combiner; such an embodiment of the prior art has themain drawback of needing a optical collimation system which is too bulkywith respect to the restricted field of view obtained.

To reduce the bulk, a combiner with optical power has been proposed;such a combiner provides its user with both the collimation of the imageand the superposition of the collimated image with the view of thescenery.

The prior art has a wealth of many and varied devices comprising acombiner with optical power. Of particular interest are the imagepresentation systems comprising a spherical concave mirror to collimatethe image.

A spherical concave mirror provides an average quality collimation of animage placed at a particular point in the space located on the axis ofthe mirror and at a distance from the latter equal to half its radius ofcurvature. By placing an imager at this point, the eye located on theaxis of a mirror receives rays coming from the imager after theirreflection from the spherical mirror; these rays are parallel and leadto the perception by the eye of a collimated image. If, furthermore, themirror is semi-reflective, it enables the same eye to observe thescenery in transmission. However in such a device the imager would haveto lie on the axis of the semi-transparent spherical mirror and it wouldblock the user's field of view.

To clear the user's view, the spherical mirror is inclined with respectto the normal to the face and the user's eye is no longer on the mirroraxis. This arrangement has the drawback of leading to a collimated imageaffected with optical aberrations, especially off-centring aberrations,which need to be corrected, at least partially.

The inclination of the spherical concave mirror impairs the collimatedimage with distortion, called off-centring distortion of the secondkind, characterized by a convergence of the verticals and an apparentcurvature of the horizontals.

The prior art teaches us to correct the distortion of the image providedby an optical assembly by introducing an inverse distortion in theimager by electronic correction; this is easily achieved when the imagerhas a cathode-ray tube but this solution is not suitable for an imager,for example a light intensifier, which does not have the required meansfor adjusting the image. It might also be possible to try to correct thedistortion by inserting another inclined spherical mirror into theoptical path between the imager and the spherical mirror, introducing adistortion which is the inverse of the first; however this would lead toan optical system which is unusable because of its bulk.

SUMMARY OF THE INVENTION

The problem consists in producing an imaging device comprising aspherical collimation mirror, having a collimated image which satisfiesthe user, i.e. an image free from annoying aberrations and having a widefield of view, preferably greater than or equal to 40 degrees. Theobject is to obtain a collimated image which has both a high resolutionand good correction of the distortion. The distortion to be corrected,due to a spherical collimation mirror observed at an oblique angle withrespect to the axis of the mirror, is an off-centred distortion of thesecond kind. The difficulty consists in finding a means to correct thedistortion without degrading the image quality, while at the same timehaving a low mass, bulk and cost and which is easy to manufacture.

For this reason, the invention proposes an optical device for a systempresenting collimated images to a user, comprising an imager and anoff-axis spherical mirror, characterized in that it comprises means tocorrect the distortion of the image presented to the user which is dueto the spherical mirror, the said means comprising a tubular concavemirror located between the imager and the spherical mirror.

The term tubular mirror refers to a mirror whose surface is generated bythe displacement of a portion of a first plane curve in translationalong a portion of a second curve. The first plane curve is preferablyin a plane perpendicular to the plane of symmetry of the system. Thesecond curve is also preferably plane and located in the plane ofsymmetry of the image presentation system. The portion of the firstplane curve is preferably a circular arc and the portion of the secondcurve as well, but it might be possible to envisage these curves beingconic sections (ellipse, parabola, hyperbola).

The surface of the tubular concave mirror provides assistance incorrecting the distortion of the image presented to the user by anoff-axis spherical collimation mirror, and, furthermore, this type ofmirror is particularly easy to machine, especially if the first curve(that which undergoes translation to generate the surface of the mirror)is a circular arc.

The device also has one or more optical power or relay groups placed onthe ray path between the imager and the spherical mirror, upstreamand/or downstream from the tubular mirror.

The correction of the distortion by a tubular mirror is in principleconsiderably better since these optical groups give the beams comingfrom the imager and incident on the tubular mirror a smaller aperture(while a large beam aperture is desirable at the spherical mirror).

These optical groups also ensure precorrection of the astigmatismnecessarily introduced into the collimated image because the sphericalmirror is observed at an angle inclined with respect to the radius whichdefines the optical axis of this mirror. This astigmatism may becorrected, for example, by a spherical convergent lens and a cylindricallens, in an optical relay group located between the imager and thetubular mirror. It may also be corrected by a diffractive lens placed ina power group between the tubular mirror and the spherical mirror.

In one particular embodiment, a power group may be provided between thetubular mirror and the spherical mirror with the following particularfeatures: it has a convergent lens whose focus is virtually centred onthe first pupil image which is the image of the pupil of the eye formedby the off-axis spherical mirror. The power group focuses the beams ofoptical rays onto the tubular mirror, which beams, coming from the pupilof the eye, have been reflected by the inclined spherical collimationmirror. These beams are almost parallel.

The invention enables a high resolution image to be maintained whileensuring substantial correction of the distortion due to the inclinedspherical collimation mirror. The invention has the advantage ofcorrecting the distortion of the image presented to the user's eye by awide instrument pupil, for example one of at least 15 millimetresdiameter, and for a wide field typically greater than 40 degrees. Theinstrument pupil is the region of space in which the user of aninstrument must place the pupil of his eye in order to use it.

This correction is particularly beneficial when a distortion cannot bereadily imposed on the imager. This is because an electronic correctionof the prior art is not suitable in such a case.

The first pupil image of the device is inclined with respect to theoptical axis, the tubular concave mirror according to the inventiongives a second pupil image of it which is rectified on the optical axis.

The invention may be integrated into a helmet sight having a wideinstrument pupil and a wide field.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will becomeapparent on reading the following detailed description of particularembodiments which are given with reference to the following appendeddrawings:

FIG. 1 shows schematically and partially an optical device with anoptical off-axis spherical combiner mirror,

FIG. 2 shows the distortion which the invention corrects,

FIG. 3 shows a device according to the invention with an optical relaysystem.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawings, the optical diagrams are shown as developed in a planecalled the plane of symmetry of the optical system. This plane containsthe normal to the entrance pupil of the user's eye and the centre of thesphere supporting the spherical mirror. In reality, the mirrors (notshown), which do not introduce any aberration, make it possible todirect the beams in three dimensions to satisfy various bulkconstraints: for example in order to adapt the device to the contour ofthe user's head.

In FIG. 1, a user of an optical device having a spherical mirror 1 isshown by the plane of the pupils 2 and the line 5 normal to this plane2. The pupil 11 of the eye is generally located optically 3 millimetresback from the cornea 12 of the eye 3.

It will be noted that, depending on its orientation with respect to theuser's face, the line 5 may correspond to the view straight in front ofthe user, or to a view which is upwards, downwards, to one side or tothe opposite side of the user.

The spherical mirror 1 is placed in front of the user, with itsconcavity turned towards the user. The intersection of the observationaxis 5 with the mirror 1 is denoted by the reference number 6.

The spherical mirror 1 is supported by a sphere whose centre 4 is notpart of this line 5. The plane of FIG. 1 is a plane in space whichcontains the centre of the sphere supporting the spherical mirror 1 andthe line 5 passing through the centre of the pupil 11 of the eye 3. Thisis the plane of incidence of the line 5 on the spherical mirror 1 and itis called the plane of symmetry of the optical system. This plane mostoften coincides with the plane passing through the centre of the pupil11 and parallel to the theoretical plane of symmetry of the user's face.

The line 5 and the radius 7 of the sphere passing through the point ofintersection 6 make an angle θ between them. A non-zero value of thisangle θ characterizes an off-axis use of the spherical mirror 1. Thespherical mirror 1 itself is called “off-axis”.

An optical ray 8 which is symmetrical with the line 5 of the opticalaxis with respect to the radius 7 of the sphere will be considered. To afirst approximation, an image whose centre 9 is placed on this opticalray at a distance equal to half of the radius of curvature of the sphereis perceived by the user's eye 3 as collimated to first order since thelight rays coming from the image thus positioned are reflected by thespherical mirror 1 in the direction of the eye 3 in the form of a beamof substantially parallel rays. The image of centre 9 may exhibit fieldcurvature.

However, the collimation by reflection from the spherical mirror is notperfect; it is affected, on top of aberrations intrinsic to this mirror,by an off-centring optical aberration due to the off-axis use of thespherical mirror 1.

The spherical mirror 1 maybe [sic] semi-transparent. In this case thelight rays 10 originating from the surroundings external to thespherical mirror 1, i.e. rays which strike the convex face of thismirror, are transmitted to the eye 3 by the spherical mirror 1. Thisspherical mirror 1 then forms a combiner which superimposes a collimatedimage on the direct view of the surroundings. It is this arrangementwhich is generally adopted in a helmet sight.

The central field is defined as the beam of light rays coming from thecentre 9 of the image to be collimated. A particular light ray whichbelongs to the central field and which passes through the centre of theuser's pupil will be considered. The path of this light ray is theoptical axis of the device used. The optical axis is generally a brokenline. The line 5 supports a part of the optical axis. The image is mostoften presented right in front of the user, the line 5 is thensubstantially normal to the user's face, but the image may, for example,be presented at the top of the user's resting field of vision atinfinity and the line 5 is then oriented in the corresponding direction.

FIG. 2 shows the image seen by the eyes of the person using an opticaldevice according to FIG. 1 in which an image, centred on the point 9 andhaving a square with a regular square grid, is collimated. The perceiveddeformation is an off-centred distortion of the second kind: thevertical lines, which should be straight parallel lines, are convergentand the horizontal lines, which should be straight parallel lines, arecurved. This distortion is due to the inclination of the sphericalcollimation mirror with respect to the observation axis.

Various optical elements will be described according to the invention inorder to obtain perception, by the user's eye, of a high-qualitycollimated image from a luminous image provided by an imager.

In FIG. 3, paths of light rays inside one embodiment of a deviceaccording to the invention are shown.

In this embodiment, intended for a helmet sight, the imager (not shown)comprises a screen, for example a cathode-ray tube screen or aliquid-crystal screen. The screen may also be formed, for example by afibre-optic bundle section or a slide or the screen of a lightintensifier tube. An image, the area of which is arbitrary, is displayedon the screen of the imager shown by its tangent plane 20. The imageprovided by the imager may be plane, spherical or even have anothershape. The paths of the light rays from the screen 20 of the imager upto the eye 3 of the user are plotted for this embodiment of theinvention.

The device has a spherical mirror 1 placed in front of the user's eye 3and a tubular mirror 21 placed between the screen 20 and the sphericalmirror 1. The device has a power group 22 between the tubular concavemirror 21 and the spherical mirror 1. It also has an optical relaysystem 29 between the screen 20 and the tubular mirror 21.

The light rays coming from the screen 20 of the imager are received,after passing through the optical relay system 29, by the tubular mirror21; they are reflected by the latter then they pass through the powergroup 22 before striking the off-axis spherical mirror 1 which ensurecollimation of the image finally perceived by the eye 3 of the user.

The light rays coming from the centre of the screen 20 of the imagerform the central field of the imager. The optical axis of the devicecorresponds to the path of the central field ray which passes throughthe centre of the pupil of the user's eye 3.

The path of light rays in the other direction, i.e. leaving the user'seye 3 and passing back through the various optical elements towards thescreen 20 of the display, is now observed.

The rays coming from the eye are reflected from the off-axis sphericalmirror 1. The optical axis which, in the example of FIG. 3, ishorizontal over a first part 31 between the centre of the pupil of theeye 3 and the spherical mirror 1 is also reflected from the sphericalmirror 1.

This part 31 of the optical axis and its reflection from the sphericalmirror 1 define a plane called the plane of incidence of the opticalaxis on the off-axis spherical mirror 1. In the example in FIG. 3, theplane of incidence is coincident with the plane of symmetry of theoptical system which is shown by the plane in FIG. 3. The plane ofsymmetry of the optical system is a plane containing the path describedby the optical axis between the imager and the user's pupil. However, anembodiment of the invention is not limited to an optical system in thisplane; within the framework of the invention, it is still possible toadd additional plane mirrors allowing, for example, optical elements tobe taken out of the plane of the figure. This is because the planemirrors, also called folding mirrors, do not modify the opticalfunction; they do not introduce or correct aberration but they do allowthe optical rays to circumvent obstacles such as the user's head.

In this embodiment example, the rays reflected by the spherical mirror 1strike a plane mirror 50 which allows the optical rays to be foldedwhile respecting the plane of incidence of the optical axis on thespherical mirror 1. The invention may be embodied without this planemirror 50. After reflection from the plane mirror 50, the optical axisis oriented along a line 32 of the plane of incidence.

Over the second part 32 of the optical axis, a first pupil image 25 isobserved, which is the image of the pupil of the eye 3 given by theoff-axis spherical mirror 1.

The normal to the plane tangent to this first pupil image 25 is notparallel to the corresponding section 32 of the optical axis. The firstpupil image 25 is inclined with respect to the optical axis. Thisinclination is an effect from the distortion to be corrected.

The power group 22 is placed, for example, so that the first pupil image25 is on the path of the light rays between the spherical mirror 1 andthe power group 22. The power group is preferably centred on the secondpart 32 of the optical axis. In this embodiment, the power groupcomprises a diffractive lens 23 and a convergent lens 24 whose focus isvirtually centred on the first pupil image 25. The diffractive lensmakes it possible for example to precorrect the astigmatism necessarilyintroduced by the off-axis observation of the spherical mirror. Thepower group may also, for example, include a divergent lens on eitherside of which are situated two convergent lenses, each one having alower optical power than the lens 24 to limit the aberrations introducedby the power group 22 itself. This optical system 22 channels the beamsof optical rays in an envelope 26, between the group 22 and the tubularmirror 21, which envelope has a substantially constant diameter. Thebeams are almost parallel in this envelope 26. The group 22 thereforereduces the aperture of the beam incident on the tubular mirror 21,which beam is upstream from this mirror 21 when considering the reversebeam paths, i.e. from the eye to the imager. This aperture is very smallin comparison with the aperture of the beams incident on the sphericalmirror 1.

The power group affects the image and it allows the optical deviceaccording to the invention to present a high-quality image. This powergroup is an optical element close to the first pupil image 25, whichhardly affects the latter.

The tubular concave mirror 21 is placed close to the second part 32 ofthe optical axis. The first pupil image 25 is on one side of the powergroup 22 and the tubular concave mirror 21 is on the other side. Thetubular concave mirror 21 is placed on the path of the rays which comefrom the pupil of the eye 3 (since the description here is described bypassing back along the actual path of the light rays coming from thescreen of the imager) and it reflects these rays towards the screen 20of the imager. The plane of FIG. 3 is also the plane of incidence of theoptical axis on the tubular mirror 21.

The useful part of the tubular mirror 21 has a tangent plane Q whosenormal 28, belonging to the plane of incidence, is not parallel to thesecond part 32 of the optical axis. The tubular mirror 21 is inclinedwith respect to the optical axis, it is called off-axis.

The tubular mirror 21 has a concave reflecting surface. This surface isgenerated by the translation of a plane curve, which is preferably acircular arc for reasons of ease of machining, along a generatrix whichis itself also a curve. In this embodiment, the generatrix is in theplane of FIG. 3 which is the plane of symmetry of the optical system ofthe device described. The plane curve which is translated is parallel toa plane perpendicular to the plane of incidence of the optical axis onthe off-axis spherical mirror 1 on one hand and containing the secondpart 32 of the optical axis starting from the tubular mirror 21 in thedirection of the spherical mirror. The second part 32 of the opticalaxis is a part of the optical axis incident on the tubular mirror 21 andsituated between the latter and the spherical mirror 1.

The translated plane curve is preferably a circular arc. The surfacethus generated is easy to machine parallel to a constant radius bytranslation of a tool driven in circular motion. The tubular mirror 21is inexpensive.

The surface of the mirror 21 enables image distortion, introduced by thespherical collimation mirror 1 used off-axis, to be corrected.

The tubular mirror 21 is close to the intermediate image 27 formed bythe device from the image displayed on the screen 20. The mirror 21hardly affects the resolution of the image but it affects the pupilimage.

The aperture around the axis 28 is sufficient to optimize the space leftavailable to place, for example, return mirrors between the tubularmirror 21 and the lens 24. The angle of incidence of the optical axisfrom the tubular mirror 21 also enables the useful surface to be limitedand thus to maintain a high-quality image over the whole surface. Theangle of incidence is preferably close to 45 degrees. In thisembodiment, the useful surface of the mirror 21 is for example estimatedto have a diameter of around 45 millimetres.

The optical device according to the invention illustrated in FIG. 3 alsoincludes an optical relay system 29 to distance the screen 20 from theimager of the tubular mirror 21. This distancing is generally madenecessary to satisfy size constraints. It makes it possible for examplefor a helmet sight to place the entire imager, which may be acathode-ray tube, in a satisfactory position within the volume availablein the helmet. The beams of the light rays between the optical relaysystem 29 and the tubular mirror 21 have a very small aperture. Thesebeams are downstream from the tubular mirror 21 when considering thereverse beam paths, i.e. from the eye to the imager. The aperture isvery small in comparison with that of the beams on the spherical mirror1. The optical relay system 29 may also have optical power functions,for example to replace the diffractive lens 23 of the group 22 and toprecorrect close to the imager the astigmatism which will be introducedby the off-axis observation of the spherical mirror 1.

In FIG. 3, the third part 33 of the optical axis which corresponds tothe reflection of the second part 32 of this same optical axis from thetubular mirror 21 is shown. The optical relay system 29 is placedbetween the tubular concave mirror 21 and the screen 20 of the imager,and it is substantially aligned with the third part 33 of the opticalaxis. This essentially centred optical relay system is simple toproduce.

In this embodiment, the optical system 29 also includes asemi-reflective plate 36, or a mixer cube which makes it possible to mixthe channel from the screen 20 with a channel from another display notshown in FIG. 3. The plate 36 makes it possible for example tosuperimpose visual information from a cathode-ray tube on informationcoming from an assembly (not shown) having a photographing objective andan image intensifier.

A second pupil image 30 may be seen on the part 33 of the optical axis,this image is located between the tubular mirror 21 and the imagerscreen 20. This second pupil image 30 is seen by the tubular mirror 21through a group 35 of lenses belonging to the optical relay system 29.

Furthermore, the magnification between the two pupil images 30 and 25 ispreferably of a value close to one. The virtually unitary pupilconjugation has the advantage of reducing the bulk of the opticaldevice, enabling the size of the optical systems throughout the opticalpath to be minimized. This reduction in bulk is advantageous both in thecase of the weight of the device and its cost.

The pupil image 30 has a tangent plane which is virtually normal to thelocal optical axis 33: this is a correction made by the tubular mirror21. This is because the first image 25 of the pupil of the eye formed bythe spherical mirror 1 is inclined with respect to the local opticalaxis 32 and corresponds to aberrations induced by this mirror 1; and thesecond pupil image 30 is rectified with respect to the optical axis 33by the tubular mirror 21 and the image is virtually perpendicular to theoptical axis 33.

The tubular mirror makes it possible for the device according to theinvention to present high pupil quality.

The off-axis spherical mirror 1 may be semi-transparent, in which casethe light rays emitted by the scenery or the surroundings within thefield of view of the user are transmitted by this mirror and arereceived by the pupil of the eye at the same time as the rays reflectedby this same mirror and previously described. The semi-transparentmirror is a combiner. It is therefore a spherical combiner usedoff-axis.

The combiner preferably forms part of a visor protecting the eyes andeven the user's face.

A visor according to the invention has at least one off-axis sphericalreflecting part. In the working position, the visor is pulled down sothat the part corresponding to the spherical mirror 1 is placed in frontof the user's eye. The entire device for presenting collimated imagesmay be integrated into a helmet, for example for a pilot of an aircraftor helicopter, making it possible to produce a helmet sight.

The sight may be monocular if it presents the collimated image just toone eye. The sight may be binocular if it involves the presentation ofan image for each eye. It has the advantage of enabling comfortablevision when the overlap of the fields of view of both images iscomplete. A binocular visor may also have a partial overlap of the twofields of view, which allows a wider field of view to be obtained forthe same optical system dimensions, without degrading the perception ofthe information presented too much.

The distortion of an image having a grid leads to the deformation of thegrid. The images which are presented to the user, and whose distortioninherent to the off-axis spherical concave visor is corrected, areparticularly advantageous in the case of a helmet sight because theyrespect the actual dimensions of the objects represented. This is ofprime importance when the sight has an image superimposed on the directview and it is even more so when the image presented replaces the directview of the user, for example in the case of night vision assisted by animage intensifier, of infrared vision or of a training simulator. Thecorrection of this distortion has the advantage of making it possiblefor the user to properly appreciate distances on the image that he isobserving and allowing him for example to pilot at night without anypositioning error.

What is claimed is:
 1. Optical device for a system presenting collimatedimages to a user, comprising an imager and an off-axis spherical mirror,characterized in that it comprises means to correct the distortion ofthe image presented to the user which is due to the spherical mirror,the said means comprising a tubular concave mirror located between theimager and the spherical mirror, the reflecting surface of the tubularmirror being generated by the displacement of a first plane curve intranslation along a second curve which is also plane.
 2. Deviceaccording to claim 1, characterized in that the spherical mirror issupported by a sphere with centre, the plane passing through the centreand through the centre of the pupil of the user's eye being the plane ofoptical symmetry of the device and the second plane curve being in theplane of optical symmetry of the device.
 3. Device according to claim 2,characterized in that it comprises one or more optical power or relaygroups placed on the ray path between the imager and the sphericalmirror, upstream and/or downstream from the tubular mirror, this orthese optical groups comprising one or more lenses to form a very smallaperture in comparison with the aperture of the beams incident on thespherical mirror.
 4. Device according to claim 3, characterized in thatat least one of the said lenses is convergent.
 5. Device according toclaim 3 characterized in that the light ray coming from the imager andpassing through the centre of the pupil of the user is the optical axisof the device, the said optical axis defining, upon its reflection fromthe tubular mirror, a first plane of incidence of the said optical axison the tubular mirror, a second plane being perpendicular to the saidfirst plane and passing through the part of the optical axis leaving thetubular mirror in the direction of the spherical mirror; the first curveplane being parallel to the said second plane.
 6. Device according toclaim 5, characterized in that the first plane curve is a circular arc.7. Device according to claim 1, characterized in that the sphericalmirror is semi-transparent.
 8. Device according to claim 1,characterized in that the system for presenting collimated images is ahelmet visor.